The moves Wednesday came a day after Spencer's agent said the seventh-year pro would have a second operation on his left knee in two months. He missed the preseason and the opening game against the Giants after the first surgery, then played against Kansas City before being sidelined again.

The 29-year-old Spencer might have played his last game for Dallas. He has a one-year deal under the franchise tag for the second straight season and will be a free agent next year.

Greenwood was a fifth-round pick by the Lions last year. He missed his rookie season with a torn abdominal muscle.